Photographic printing apparatus



Feb. 9, 1954 E. c. ROGERS, JR 2,568,474

PHOTOGRAPHIC PRINTING APPARATUS Filed March 28, 1949 .14 INVENTOR.

' E4 waagcpaasnsk o dark portions. If desired, the lamp 23 and its housing may be omitted and the position of the light-responsive device selected by the location of the cathode-shadow with respect to the image appearing on the ground-glass 22. In either arrangement, the desideratum is that the light condensed by the lens 2| on the cathode of the photoelectric cell 20 comes from an area of pre determined average tone quality in the image projected by the lens I3 of the enlarger. It is also desirable that in any one apparatus the axis of the light-responsive device be at a fixed angle to the surface of the support It; for the intensity of light reflected from the surface of the sensitized paper will depend upon the angle of reflection.

ihe light-responsive unit above described is employed to control the operation of the timing apparatus illustrated in Fig. 2. Such apparatus comprises a pair of electron discharge devices, such as the pentodes 3t and 3|, the cathode heating filaments of which are supplied with current from a step-down transformer 32 the primary of which is connected across an alternating-currentsupply line 3334, conveniently providing the common 110-volt, 60-cycle current. One of the supply conductors, shown as the conductor 33, is directly connected to the cathode of the pentode 3G and connected to the cathode of the pentode 3| through a variable resistance 35. The anode or plate of the pentode 3! is connected to the other supply conductor 34 through the winding of a holding relay 38 and -a fixed resistance 39. The plate of the pentode 33 is connected to the conductor 34 through the resistance 39, a second fixed resistance 40, and a potentiometer 4| having an adjustable tap 42. The control grid of the pentode 3! is directly connected to the plate of the pentode 30, while the control grid of the pentode 33 is connected to one terminal of a timing condenser 43. i

The projecting lamp I is under the control of a relay 45 having a movable contact 45 and a fixed contact 4'11 separated from each other when the relay is de-energized. ,One terminal of the winding of the relay 45 is connected to the supply conductor 34, while the other terminal is connected to the supply conductor 33 through the normally separated contacts of the holding relay 38 and also through a normally open, push-button switch 48 in parallel with the contacts of the relay 38. To permit use of the projecting lamp It for set-up purposes, a manually controlled switch 49 may be connected in parallel with the contacts 46 and 41' of the relay 45.

As previously indicated, one terminal of the timing condenser 43 is connected to the control grid. of the pentode 30. The other terminal of such condenser is connected to the movable contact of a single-pole, double-throw switch the two stationary contacts 52 and 53 of which are connected respectively to the tap 42 of the potentiometer 4i and to the cathode of the pentode 35. The dotted line 54 in Fig. 2 indicates a mechanical interconnection between the movable contact 5| and the movable contact 46 of the relay 45, the arrangement being such that the movable contact 51 engages the fixed contact 52 when the relay 45 is de-energized and engages the fixed contact 53 when the relay 45 is energized.

Connected in parallel across the resistances 40 and 4| is a potentiometer 56 having an adjustable tap 51 connected to the anode of the photoelectric cell 20. The cathode of the photoelectric cell is connected. to the control, grid of the pentode 4 30. For a purpose which will hereinafter become apparent, a manual, normally open push-button switch 58 is connected across the terminals of the timing condenser 43.

The lamp 23 associated with the light-responsive element is shown in Fig. 2 as connected to the supply conductors 33 and 34 through a switch 50. The projecting lamp in is connected to the supply wires 33 and 3 5 through the parallel switches 4E4l and 49.

The parts are illustrated in Fig. 2 in the condition existing during the interval between exposures. In this condition, the relays 45 and 38 are de-energized, and the i'nova-ble contact 5! is in engagement with the fixed contact 52 to connect the condenser 23 between the grid of the pentode 30 and the adjustable tap 42 of the potentiometer 4!. Since the plates of both pentodes are connected to the same supply conductor 34 both pentodes will be non-conductive during one-half of each cycle of the supply current; and so far as operation of the pentodes is concerned, they may therefore be regarded as subjected to a directcurrent plate voltage. The pentode 33 will be conducting, depressing the potential of its plate and that of the grid of the pentode 31. The resistance 33 is great enough to insure that the grid of pentode 3| will possess a negative bias great enough to render that tube non-conductive, and thus maintain the relay 38 de-energized as long as the tube 38 is conductive.

It will be clear from Fig. 2 that in the condition there illustrated the potential difference between the control grid of the pentode 30 and the tap 42 of the potentiometer 4| will be applied across the timing condenser 43 to impart a charge thereto and that the potential to which the timing condenser is charged will depend upon the position of the tap 42. Assuming that the apparatus has been adjusted to take account of the printing qualities of the paper being employed, that the light-responsive device 2ll--2l has been properly positioned, and that everything is in readiness for the making of an enlargement, printing operation is started by closing the pushbutton switch 48. This operation completes a circuit through the winding of the relay =45, closes the switch 45-41, and causes the projecting lamp [0 to light. Energization of the relay 45 also moves the movable contact 5| from engagement with the contact 52 into engagement with the contact 53. Such movement of the contact 5| connects the timing condenser 43 between the cathode and grid of the pentode 38 thereby rendered non-conductive. This causes an immediate rise in the potential of the plate of the pentode 30 and of the grid of the pentode 3|. The pentode 3i thereupon becomes conductive, and the resultant current flowing through the relay 38 causes the contacts of such relay to close and make it possible for the operator to release the push button switch 48 in parallel with such contacts, the relay 45 remaining energized and the projection lamp H] in operation. The condenser 43 now discharges through the photoelectric cell at a rate dependent upon the amount of light received by such cell; and as the condenser discharges, the negative bias of the grid of the pentode 30 decreases. Eventually, such bias becomes small enough to render the pentode 3E} conductive, whereupon the potential of its plate and of the grid of the pentode 3| decreases, the pentode 3| becomes non-conductive, the relay 38 is deenergized to de-energize the relay 45, and the contacts 46 and 41 of the latter relayseparate to extinguish'the projecting lamp ln' and terminate the printing operation.

The'position of the tap 42 along the'resistance 4| determines the charge impressed on the timing condenser 43 :during intervals between exposures, and the tap 42 may therefore be adjusted to suit the speed of the sensitized paper being used in the production of enlargements. In order to prevent the charge impressed on the condenser 43 during one setting or the tap 42 from influencing operation of the device after readjustment of such tap, the push-button switch '58 is momentarily :closed to discharge the condenser 43 after each tap-readjustment.

It will be noted that the potentiometer 56 forms-part of a circuit which includes the timing condenser 43 and photoelectriccell '23 and which is in parallel with the pentode 30. During a timed exposure, the voltage drop across this potentiometer will be slowly increasing as the condenser 43 discharges, the rate of change in such voltage drop depending upon the intensity of light striking the photoelectric cell-and upon the-setting of the tap 51. As a result, the positive potential applied to the anode of the photoof'change of the potential applied to the anode of the photoelectric cell will increase as the tap .51 is moved toward that terminal of the resistance 56 which is connected to the plate of the pentode. The greater the change in potential applied to the anode of the photoelectric cell, the longer will be the long exposures as compared with the short exposures, and vice versa. Hence, the potentiometer 55 enables the operator to control the slope of the curve representing the relation between light-intensity and exposure-time and thus compensate for such variables as grid current, condenser leakage, wiring leakage, and gas currents in the photoelectric cell and in the pentode 30.

The variable resistance 36 controls the oathode-potential, and hence the effective grid potential, of the pentode 3|. Its adjustment provides large current and voltage changes for the pentode 3| and can be employed to effect large changes in light-time values by shifting the operating characteristics of the circuit.

Desirably, the heating-filament circuit of the pentode 30 includes a resistance 62 which maintains filament heat, and thus grid and gas currents in the pentode 30, at a low value so as to reduce the magnitude of the factors compensated for by adjustment of the potentiometer 56.

To increase the range of adjustment it may be desirable in some instances to vary the capacity of the timing condenser. To this end, a second condenser 43' may be connected in parallel with the condenser 43 through a switch 63 which can be closed or opened as necessary to increase or decrease the effective capacity of the timing condenser. If the voltage across the supply conductors 33 and 34 is subject to variation, it is desirable to employ a voltage-regulator tube 64 connected across such conductors. The degree of voltage provided by the tube 64 is controlled by the value of the resistance 39.

It is common in photo-enlarging work to employ a so called safe light which projects light of a wave length which is visible but to which printing papers are not responsive. In ordinary photoelectric cell 20 might be. trol of the safe light may be conveniently efenergized. relay is energized to institute a printing operawork, such a safe light can be left on during a printing operation. With apparatus embodying my invention, howevenitis advisable to exfected through the expedient illustrated in Fig. 2, where a safe light 19 is shown as connected to the supply conductors 33 and 34 through a switch comprising the movable-contact '46 of the relay 4'! and a fixed contact H which such movable contact engages when the relay 45 is :de-

With this arrangement, when the tion, the safe light is automatically turned out as a result of operation of the contacts 46 and To set the device up {or operation, a negative is positioned in the enlarger,the projection lamp turned on by closing the manualswitch 43,-and the enlarger is adjusted to project onto the surface of the support 18 a focused image of the desired size. The light-responsive control device is'then positioned with respect to'the-image in the manner above described so that reflected light from an image-portion of the predetermined tone quality will be'p'rojected onto the cathode of the photoelectric cell 20. If the tap 42 of the potentiometer 4| is not,properlyadjustedrorthe speed of the printing paper to 'be used, its adjustment is appropriately changed and thepushbutton switch 58 momentarily'closed todischarge the condenser 43 and thus insure that when recharged its potential will correspond to the new setting of the tap 42. With themanual switch 45 opened to extinguish the projecting lamp, the apparatus is in condition for the making of an exposure. With the aid of illumination provided by the safe light, .which is connected to the supply conductors 33 and 34 through the interengaged contacts 46 and H, the operator positions the printing paper on the support It and momentarily closes the push-button switch 48 whereupon the apparatus operates automatically to extinguish the safe light 10 and initiate an exposure by turning on the projection lamp Ill. Upon termination of the exposure period, marked by the de-energization of the relays 38 and 45 as above set forth, the projection lamp I0 is extinguished and the safe light 10 again turned on.

In making a plurality of enlargements from a single negative, no further adjustment of the apparatus is required. It is only necessary to remove the exposed paper, position the new sheet, and again close momentarily the push-button switch 48. The minimum possible interval between exposures is much greater than that necessary to charge the condenser 43 fully. A change in negatives will ordinarily require a change in the positioning of the light-responsive control device, and a change in paper used may require a readjustment of the tap 42.

The only movable parts of the timing circuit which need be exposed for ordinary manual manipulation are a control for the tap 42 and the operating members of the push-button switches 48 and 58. The adjustments for the variable resistances 36 and 56 need not be, and. desirably are not, readily accessible; for those adjustments need be made only at infrequent intervals, as to compensate for aging of the tubes.

I am aware that it is old to time intervals by the discharge'of a condenser associated with an electron discharge device, such as the pentode 3D. In many of such prior devices, however, it is necessary to perform some manual operation, such as the opening of a switch, after the termination of the automatically timed interval in order to prevent the condenser from recharging and automatically instituting a new operation. When such a device is used to control a printing exposure, adventitious recharging of the condenser would result in a re-exposure which might be highly undesirable. No such adventitious re.- exposure can occur in the use of my device; since the operation of the switch 5| and the control of an automatically timed interval prevents the recharging of the condenser from instituting a new operation.

While the apparatus above described is primarily suited for the timing of exposures in photo-enlarging, the circuit illustrated in Fig. 2 can be used in other situations, as the source of light which controls the photo-electric cell 20 is immaterial. It will also be understood that while I have illustrated in Fig. 2 a circuit specifically adapted for connection to an alternating-current supply the apparatus can be adapted, by obvious changes, for use with a direct-current supply.

I claim as my invention.

1. In an automatic apparatus for timing an exposure in photo-enlarging, a light-responsive element positionable to receive light reflected at a predetermined angle from a selected portion only of the sensitized surface being exposed, automatic means controlled by said element for terminating the exposure, a lens for concentrating on the light-responsive element light reflected from the selected portion of the sensitized surface, and a translucent screen positioned on the opposite side of said light-responsive element in position to receive in superposed relation an image of the selected portion of the sensitized surface and the-shadow of the light-responsive element.

2. In an automatic apparatus for timing an exposure in photo-enlarging, a light-responsive element positionable to receive light reflected at a predetermined angle from a selected portion only of the sensitized surface being exposed, automatic means controlled by said element for terminating the exposure, a lens for concentrating on the light-responsive element light reflected from the selected portion of the sensitized surface, and means co-operating with said lens for projecting on to the sensitized surface a silhouette image of said light-responsive element, said last named means comprising a light-source located on the opposite side of the light-responsive element from the lens, and a translucent screen located between such element and the light-source.

ELWOOD C. ROGERS, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,000,589 Fuller May '7, 1935 2,090,224 Noble Aug. 17, 1937 2,201,606 Bing May 21, 1940 2,282,045 Fleischer May 5, 1942 2,350,831 Schubert June 6, 1944 2,444,675 Rath July 6, 1948 2,469,076 Rabinowitz May 3, 1949 2,484,299 Lebrum Oct. 11, 1949 

